The Breakdown Diode
The breakdown mechanism is the Zener effect (tunneling) for abrupt junctions with extremely heavy doping; however, the more common breakdown is avalanche (impact ionization), typical of more lightly doped or graded junctions.
The breakdown diode:
- By varying the doping we can fabricate diodes with specific breakdown voltages ranging from less than one volt to several hundred volts.
- If the junction is well designed, the breakdown will be sharp and the current after breakdown will be essentially independent of voltage (Fig. a).
- When a diode is designed for a specific breakdown voltage, it is called a breakdown diode.
- Such diodes are also called Zener diodes, despite the fact that the actual breakdown mechanism is usually the avalanche effect.
- This error in terminology is due to an early mistake in identifying the first observations of breakdown in p-n junctions.
- Breakdown diodes can be used as voltage regulators in circuits with varying inputs.
- The 15-V breakdown diode of Fig. holds the circuit output voltage v0 constant at 15 V, while the input varies at voltages greater than 15 V.
For example, if vs is a rectified and filtered signal composed of a 17-V d-c component and a 1-V ripple variation above and below 17 V, the output
v0 will remain constant at 15 V.
- More complicated voltage regulator circuits can be designed using breakdown diodes, depending on the type of signal being regulated and the nature of the output load.
- In a similar application, such a device can be used as a reference diode; since the breakdown voltage of a particular diode is known, the voltage across it during breakdown can be used as a reference in circuits that require a known value of voltage.